The proposed research analyzes processes underlying changes in infants' attentional responses as a function of experience with a stimulus. Previous analyses have emphasized a cognitive-memorial process responsible for decrements in responding to a redundant stimulus (habituation). The current proposal centers on a motivational-arousal process responsible for transient increases in responding to effectively intense stimuli (sensitization). A dual-process analysis is advanced according to which an infant's responses to repeated stimulation are the outcome of the interaction between two fundamental learning processes, habituation and sensitization. Three main avenues of research are proposed. The first focuses on the generality of dual-process analysis by applying it to infants' reactions to naturalistic face stimuli. The main question addressed is whether evidence exists to show that faces, in addition to their other roles in infant attention, also sensitize infant visual fixation. The second line of research continues past investigations of the stimulus determinants of sensitization. Stimulus parameters such as the suprathreshold contrast in the stimulus and its chromatic characteristics will be explored. The visual capabilities of different aged infants will be estimated by performing experiments on contrast in the stimulus and its chromatic characteristics will be explored. The visual capabilities of different aged infants will be estimated by performing experiments on contrast sensitivity, luminosity functions, and cone action spectra. This research will include developmental analyses, studies on individual differences in vision and how they relate to individual differences in pattern preferences, and studies with preterm infants. The third research avenue focuses on the consequences of sensitization for information- processing in infants. Sensitization facilitates information-processing in at least two ways: (1) Sensitizing stimuli transfer their arousing properties to other stimuli that reliably predict them, and (2) sensitizing signals lead to improved prospective processing of either the elements in an associative learning situation, or of the relation between the elements. By combining cognitive-memorial and motivational-arousal processes in one theory, the dual-process analysis holds promise as a more comprehensive treatment of infant attentional dynamics. The dual-process account has implications for the understanding of "special" populations of infants, who differ in their "arousability" relative to healthy, full-term infants.